The present invention relates to etch baths for etching insulative and conductive layers on a semiconductor wafer substrate in the fabrication of integrated circuits on the substrate. More particularly, the present invention relates to a wet etch system which utilizes an overflow of etchant from the bath before each etching cycle in order to remove potential device-contaminating particulate precipitates from an etch bath.
Generally, the process for manufacturing integrated circuits on a silicon wafer substrate typically involves deposition of a thin dielectric or conductive film on the wafer using oxidation or any of a variety of chemical vapor deposition processes; formation of a circuit pattern on a layer of photoresist material by photolithography; placing a photoresist mask layer corresponding to the circuit pattern on the wafer; etching of the circuit pattern in the conductive layer on the wafer; and stripping of the photoresist mask layer from the wafer. These steps are carried out in highly-specialized, automated equipment. Many of the steps used in the IC fabrication process, particularly the etching and photoresist stripping steps, provide abundant opportunity for potential circuit-contaminating particles to form and contaminate the devices being fabricated on the wafer.
In the semiconductor fabrication industry, minimization of particle contamination on semiconductor wafers increases in importance as the integrated circuit devices on the wafers decrease in size. With the reduced size of the devices, a contaminant having a particular size occupies a relatively larger percentage of the available space for circuit elements on the wafer as compared to wafers containing the larger devices of the past. Moreover, the presence of particles in the integrated circuits compromises the functional integrity of the devices in the finished electronic product. Currently, mini-environment based IC manufacturing facilities are equipped to control airborne particles much smaller than 1.0 xcexcm, as surface contamination continues to be of high priority to semiconductor manufacturers. To achieve an ultra clean wafer surface, particles must be removed from the wafer, and particle-removing methods are therefore of utmost importance in the fabrication of semiconductors.
The most common system for cleaning semiconductor wafers during wafer processing includes a series of tanks which contain the necessary cleaning solutions and are positioned in a xe2x80x9cwet benchxe2x80x9d in a clean room. Batches of wafers are moved in sequence through the tanks, typically by operation of a computer-controlled automated apparatus. Currently, semiconductor manufacturers use wet cleaning processes which may use cleaning agents such as deionized water and/or surfactants. Other wafer-cleaning processes utilize solvents, dry cleaning using high-velocity gas jets, and a megasonic cleaning process, in which very high-frequency sound waves are used to dislodge particles from the wafer surface. Cleaning systems which use deionized (DI) water currently are widely used in the industry because the systems are effective in removing particles from the wafers and are relatively cost-efficient. Approximately 4.5 tons of water are used for the production of each 200-mm, 16-Mbit, DRAM wafer.
Etching processes are used to form the geometric circuit patterns in the layers and to form vias for electrical contact between the layers. Etching processes include xe2x80x9cwetxe2x80x9d etching, in which one or more chemical reagents are brought into direct contact with the substrate, and xe2x80x9cdryxe2x80x9d etching, such as reactive ion (RI) etching, reactive ion beam etching and plasma etching. In plasma etching processes, a gas is first introduced into a reaction chamber and then plasma is generated from the gas. This is accomplished by dissociation of the gas into ions, free radicals and electrons by using an RF (radio frequency) generator, which includes one or more electrodes. The electrodes are accelerated in an electric field generated by the electrodes, and the energized electrons strike gas molecules to form additional ions, free radicals and electrons, which strike additional gas molecules, and the plasma eventually becomes self-sustaining. The ions, free radicals and electrons in the plasma react chemically with the layer material on the semiconductor wafer to form residual products which leave the wafer surface and thus, etch the material from the wafer.
The plasma generated in a plasma etching process includes high-energy ions, free radicals and electrons which react chemically with the surface material of the semiconductor wafer to form reaction produces that leave the wafer surface, thereby etching a geometrical pattern or a via in a wafer layer. Plasma intensity depends on the type of etchant gas or gases used, as well as the etchant gas pressure and temperature and the radio frequency generated at an electrode in the process chamber by an RF generator. If any of these factors changes during the process, the plasma intensity may increase or decrease with respect to the plasma intensity level required for optimum etching in a particular application. Decreased plasma intensity results in decreased, and thus incomplete, etching. Increased plasma intensity, on the other hand, can cause overetching and plasma-induced damage of the wafers. Plasma-induced damage includes trapped interface charges, material defects migration into bulk materials, and contamination caused by the deposition of etch products on material surfaces. Etch damage induced by reactive plasma can alter the qualities of sensitive IC components such as Schottky diodes, the rectifying capability of which can be reduced considerably. Heavy-polymer deposition during oxide contact hole etching may cause high-contact resistance.
Furthermore, plasma-etching techniques are incapable of discriminating between the layer or layers to be etched and the underlying layer or layers, which should remain unaffected by the etching process. For these reasons, the plasma reactor must be equipped with a monitor that indicates when the etching process is to be stopped. Such a monitor may utilize an end-point system or mode to terminate etching in order to prevent undesired etching of the underlying layer on the wafer.
Dry etching methods, particularly plasma etching, have become the most widely-used methods for etching wafers in advanced IC wafer fabrication. Materials such as silicon and aluminum are nearly always dry etched for submicron fabrication. However, in some cases, wet etching processes are capable of providing high etch selectivity that cannot be attained using dry etching. Moreover, wet etching eliminates plasma damage. A wet etching process which utilizes hot phosphoric acid (H3PO4) is frequently used to remove masking layers made of silicon nitride (Si3N4). The wet etching process is typically carried out by immersing successive batches of wafers in the heated liquid etchant phosphoric acid, which is contained in a bath container.
During the wet etching process, the etchant phosphoric acid reacts with the silicon nitride to form silicon dioxide as a byproduct. A common drawback of conventional wet etching systems is that the silicon nitride precipitate tends to accumulate in the bath container and contaminate devices being formed on the wafer. Many conventional wet etching systems include a filter loop having a filter pump which removes the particulate contaminants from the etchant, after which the filtered etchant is returned to the etch bath. However, over time the filter gradually becomes clogged and less efficient in the removal of the particles from the etchant in the etch bath. Accordingly, a new and improved etch bath is needed for the efficient and frequent removal of particulate contaminants from an etch bath between etch cycles.
An object of the present invention is to provide a new and improved wet etch system for preventing particle contamination of devices being formed on a wafer substrate during a wet etching process.
Another object of the present invention is to provide a new and improved wet etch system which removes potential device-contaminating particles from an etch bath between wet etching cycles.
Still another object of the present invention is to provide a wet etch system which utilizes a regular overflow of used etchant from an etch bath to remove potential device-contaminating particles from the bath prior to each subsequent wet etch cycle.
Yet another object of the present invention is to provide a wet etch system which includes a process tank having an inner etch bath chamber and an outer overflow chamber surrounding the inner etch bath chamber for receiving overflow of etchant from the inner etch bath chamber upon placement of a wafer-containing cassette in the etch bath container for a subsequent cycle of wet etching.
A still further object of the present invention is to provide a wet etch system including a process tank which is divided into an inner etch bath chamber, an outer overflow chamber for receiving an overflow of etchant from the etch bath chamber upon placement of a wafer-containing cassette in the etch bath chamber, and a water spray loop provided in the overflow chamber for rinsing overflow etchant down the walls of the overflow chamber.
The present invention is generally directed to a wet etch system which includes a process tank having an inner etch bath chamber for containing a liquid etchant and an outer overflow chamber surrounding the etch bath chamber. A frame which is typically removably mounted on the process tank defines a diversion channel between the upper ends of the etch bath chamber and overflow chamber. In operation, the etch bath chamber receives a wafer-containing cassette, which displaces etchant from the etch bath chamber, through the diversion channel and into the overflow chamber, where the etchant is drained from the process tank through drain openings in the bottom of the overflow chamber. Particulate impurities which formed in the etch bath chamber during previous wet etching cycles leave the etch bath chamber, enter the overflow chamber and are drained from the process tank with the overflow etchant. Finally, fresh etchant is poured into the etch bath chamber prior to a subsequent etch cycle. A water spray loop may be provided in the overflow chamber for spraying and removing used etchant and etch particles from the interior wall surfaces of the overflow chamber during or after each etch cycle.